1. Field of the Invention
The present invention relates to an optical disc drive apparatus and particularly to a reading optical system.
2. Background Art
The capacity of a single layer in an optical disc largely depends on the wavelength of the semiconductor laser used and the numerical aperture (NA) of the objective lens. The shorter the wavelength of the semiconductor laser, or the larger the NA, the greater the recording density will be, with a resultant increase in the capacity of each layer. Most of the currently commercially available optical disc drives are DVD (Digital Versatile Disc) drives that employ the color of red with wavelengths in the vicinity of 660 nm and an objective lens having an NA of 0.6. Shipping has started of optical drives that exceed the recording density of DVDs, using a light source consisting of a semiconductor laser of blue-violet light with wavelengths in the vicinity of 405 nm and an objective lens with an NA of 0.85. Difficulty is expected for the development of a semiconductor laser light source having wavelengths shorter than those of such blue violet because the wavelengths would be in the UV range. Furthermore, since the limit of NA of an objective lens in air is 1, improvement in recording density by means of the objective lens is also becoming difficult.
Under such circumstances, use of multiple layers is suggested as a means of increasing the capacity of an individual optical disc. For example, Non-patent Document 1 discloses a ROM (Read Only Memory) having four layers. When a multilayer optical disc is irradiated with laser light, crosstalk between the layers becomes an issue because of the simultaneous irradiation of a plurality of layers. In order to address this problem, the interlayer distance is increased. In this way, crosstalk can be reduced because laser light is focused and layers other than a target layer are displaced from the position where the laser light is focused.
However, such increase in the interlayer distance gives rise to the problem of spherical aberration. Between the recording layers, polycarbonate is used, which has a refractive index different from that of air and thus poses a cause for spherical aberration. The objective lens is designed such that its spherical aberration is minimized with respect to a particular layer. As a result, spherical aberration is caused when the focus of laser light is shifted to any of layers other than the target layer. Such aberration can be normally corrected by placing an expander lens system consisting of two lenses in front of an objective lens. The aberration can also be corrected by varying the phase of a liquid crystal element or the distance between two lenses. However, it is impossible to correct large spherical aberration, given the possible range of compensation of the liquid crystal element or the need to realize a lens transfer mechanism within the small optical disc drive apparatus. Thus, it is difficult to achieve a sufficient increase in the interlayer distance in a multilayer optical disc for actual optical drive units. Consequently, some interlayer crosstalk inevitably remains in a multilayered optical disc.
In order to reduce the aforementioned crosstalk, in Patent Document 1, a minute mirror is disposed on the optical axis so as to obtain only the reflected light of interest and reduce crosstalk. This takes advantage of the fact that the position along the optical axis where the reflected light from a multilayer optical disc is focused by lenses differs between the reflected light from a target layer and that from an adjacent layer. In Non-patent Document 2, in order to reduce crosstalk from an adjacent layer, the reflected light from the multilayer disc is focused by a condenser lens. Two split wave plates consisting of a phase difference region of a + quarter-wave plate and a phase difference region of a − quarter-wave plate are disposed such that their directions are inverted with respect to each other, with the position of focus placed between them. Because the focal point of the reflected light from the target layer is sandwiched between the two split wave plates, the light only passes through either the plus λ/4 region or the minus λ/4 region of the two split wave plates twice, thus producing a phase difference λ/2 between the two polarization directions and rotating the polarization directions by 90°. The reflected light from an adjacent layer has its focus position located outside the two wave plates, so that it passes through both the plus λ/4 region and the minus λ/4 region. In this case, the phase difference caused by the split wave plates is cancelled and no phase difference is produced between the two polarization directions. Thus, the polarization direction of the reflected light from the adjacent layer is not changed. Such separation of polarization makes it possible to obtain the reflected light only from the target layer, so that crosstalk from an adjacent layer can be reduced. In this method, however, an optical element for polarization separation needs to be introduced into the optical system, resulting in an increase in the size of the optical system.
Patent Document 1: JP Patent Publication (Kokai) No. 2005-302084 A
Non-patent Document 1: Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 778-783
Non-patent Document 2: Optics Japan 2005, 23aPD1